The effects of hydrogen absorption and desorption in Pd films are studied through reflectivity and conductance measurements. The data show that there is an appreciable variation in both signals even when the sample is exposed to a low hydrogen concentration that does not induce the αβ transition. The reflectivity variations are explained in terms of a rise of the Fermi level in the bulk of the Pd film, while the conductance decrease in a hydrogen atmosphere is related to the behaviour of scattering centres of the absorbed hydrogen atoms. Such ‘bulk’ effects are correlated to the flat-band voltage shift measured in Pd-gate MOS capacitors and two contributions to this quantity are isolated by a temporal analysis of the processes. A model based on two different sites for the absorbed hydrogen atoms is proposed to explain the results.